I ran into some trouble making fire strikers work properly late this summer.
I have been making them for a long time and all of a sudden they quit
working. Using brand new 3/16" X 1/4" W1 from MSC. The trouble SEEMED to
start when I got into a new 3 foot bar.
After a lot of question-asking and some research and analysis, I think I
have solved my problem.
What do you guys know about decarburization of tool steels?
I'm not telling what I think I learned until I hear from some of you.
Because the carbon goes away. In general, it's the result of
heating carbon steel in air -- the hotter you get the steel, the worse
the condition is. Heating it above transition temperature (Curie
point) without protection from oxygen in the air will start decarb
immediatelyl. The carbon combines with the oxygen. The higher the temp
and the longer the part soaks at high temperature, the worse it gets.
Plain high-carbon steels (W1; music wire; AISI/SAE 1070 and above) are
the worst offenders. High-alloy steels, in general, present less of a
An eigth-inch layer of bark (decarb layer) is quite possible if you
really screw it up.
When they used to make a lot of punches and other tools from W1, it
was a major heat-treat issue. The popular solutions are heating in a
carbon boat (for small-scale heat treating); heating in a dissociated
ammonia or other carbon-rich atmosphere; heating in a ceramic, clay,
or steel boat with charcoal or carburizing compound; and heating in a
vacuum. Look up "muffle furnace," too.
Oh, I forgot: Wrapping in stainless steel foil is commonly done for
onesies and twosies of alloy steels, but I don't know if it's
effective enough for plain-carbon steel. Maybe.
It's an issue in blacksmith work with a coal forge, too. You need to
keep the air blast from playing directly on the steel.
It works for O-1. I smear some Ivory soap on the steel and add charcoal to
the pouch for good measure. The cutting tool comes out light grey after
quenching and cuts relatively hard steel like truck springs with only
minimal grinding or honing.
Tin can steel works pretty well if you don't have the stainless foil. I
leave one end open and cram in charcoal so the tool will shake out easily
into the quench pot.
It provides a little carbon to help prevent scale and it probably
helps prevent decarb, as well.
Just be aware that charcoal briquettes put out VOLUMINOUS amounts of
carbon monoxide when heated. You might want to put a CO detector
nearby -- not that one briquette is going to flood the room with CO,
but you'll want to be aware of any concentration around the furnace.
In commercial heat-treating, CO is a commonly used source of
atmospheric carbon. Where they want to simply avoid oxygen, big
continuous-process heat-treat muffle furnaces often dissociate
ammonia (NH3) and pump the combined N and H gases into the furnace to
purge oxygen. Whether they use a CO atmosphere or a N/H atmosphere,
the overflow is burned off at the furnace outlet to prevent explosions
from CO or H2.
So you can avoid changing the carbon content of the steel either by
purging oxygen, or by "doping" the atmosphere with some carbon to
combine with any oxygen that's present in the atmosphere. Obviously,
getting a neutral result that neither adds nor subtracts carbon from
the steel is a tricky proposition with the carbon-rich atmosphere.
An animal fat based soap bakes into the hard protective crud layer that's so
difficult to scrub off a barbecue, though quenching from red heat knocks it
right off. As Boy Scouts we learned to rub Ivory on the bottom of cooking
pans so the soot from the campfire would wash off easily.
I've read that a paste of flour and salt works too.
[ ... ]
Include a layer or two of paper inside the SS foil, and crimp it
hard enough to keep air from getting in easily. If there are trapped
air spaces in the foil wrap, add more paper. The paper burns using the
oxygen and leaves a carbon source in contact with the steel.
Makes sense. The coal should contribute more carbon, if the
workpiece is kept in contact with the coal.
And, of course, if you have a small electric furnace, *and* a
TIG setup, flow argon (or some other inert gas) into the oven to push
out the air with its oxygen.
Right on the money, EH. :)
Yeah but. LOL :)
Except high alloy steels have the worst reputation for reasons you
gave in the first part tho. The high alloy steels need longer soak
times at higher temperatures and so cook off more of their carbon.
W1 etc many times can be heated quickly and quenched.
But yeah, even heated and quenched quickly there's still a reduced
carbon layer of some sort when using my propane burners. In my case
it's expected and I just grind it off but also I'd grind it thinner
anyway since a thin knife blade cuts most stuff better. ;)
With my gun and knife springs made from 1095 or O1 the thin reduced
carbon layer doesn't seem to effect anything. ?? :)
Alvin in AZ
Hmmm...Ok. That could get unnecessarily complicated for what's being
discussed here, but, in a broad generality, the diffusion rate of
carbon in plain-carbon steel is higher than for many, if not most
alloys, and thus it reaches the surface and oxidizes more quickly at
high temperatures. It's also true that alloy steels containing
chromium, molybdenum, and maybe some other elements require less
carbon to achieve a given hardness, so, in terms of hardenability,
it's a more critical issue with plain-carbon steel.
Beyond that, it's probably not important to get into the details.
True. You sacrifice some strength if you also temper it quickly, but
it's done all the time in blacksmith work and in other heat treating
done in a forge or with a torch, as a practical matter. And the
initial heating/quenching rate itself doesn't have much effect as long
as the piece has reached the transition temperature all the way
It affects something, but not something that may be noticed or that
may matter in your applications.
A skin of decarb reduces the yield point and hardness of that surface
layer, right where it matters most. However, it doesn't affect the
Young's modulus, so it has no effect on spring rate -- within the
So a spring with a very thin decarbed surface will perform
identically, as a spring, to one that's hard all the way through. It
just can't be bent quite as much without taking on some degree of
permanent set. In a normal application, your spring probably is
operating well within the yield-strength envelope of that steel, so
you'll never notice it.
Contact is not really needed. There is gas between the coal chunks, and
where you are in the fire determines if there's (much) oxygen in that
Nitrogen is way cheaper. AFAIK, most electric kiln/furnace elements
(Kanthal or the like) depend on an oxide coating, so you may burn them
out faster that way - not a problem with the stainless foil envelope and
a bit of something to burn inside the envelope to use up oxygen/supply
You can't see it directly in a forge (all that coal gets in the way) but
it's similar to an oxy/acetylene or propane/air flame in that there's
fuel-rich and oxygen rich areas. You want to heat the steel in a
fuel-rich area (not really the coal, as such, but the gases given off
from the coal.) When doing small things the expensive way (OA) you use a
"soft" or carburizing (fuel-rich) flame with a long feather. With a coal
forge, if you get the steel too deep in the fire, or don't build the
fire up with enough coal, or use to much air blast, you burn out the
carbon by having the steel hot (for a prolonged time) in an oxygen-rich
environment. It's obviously going to see a little of that when it's out
of the fire and hot, but not for such a long time.